Enhanced field emission from carbon nanotubes mixed with particles

ABSTRACT

The present invention is directed toward cathodes and cathode materials comprising carbon nanotubes (CNTs) and particles. The present invention is also directed toward field emission devices comprising a cathode of the present invention, as well as methods for making these cathodes. In some embodiments, the cathode of the present invention is used in a field emission display. The invention also comprises a method of depositing a layer of CNTs and particles onto a substrate to form a cathode of the present invention, as well as a method of controlling the density of CNTs used in this mixed layer in an effort to optimize the field emission properties of the resulting layer for field emission display applications.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/417,246, filed Oct. 9, 2002.

TECHNICAL FIELD

The present invention relates in general to field emission devices, andin particular to field emission devices comprising carbon nanotubes.

BACKGROUND INFORMATION

Carbon nanotubes (CNTs) have intriguing physical and chemical propertieswhich have consequently made them the object of numerous ongoing studies(Ajayan et al., Top. Appl. Phys., vol. 80, p. 391, 2001; Dai, Acc. Chem.Res., vol. 35, p. 1035, 2002). As a result of some of these studies,carbon nanotubes have been found to be excellent cathode materials forfield emission displays because of their high aspect ratio andoutstanding chemical inertness (U.S. Pat. No. 5,773,921). Single-wallcarbon nanotubes (SWNTs) are hollow carbon fullerene tubes that havediameters from 5 angstroms to several nanometers (nm) and can be microns(μm) long or longer. Multi-wall carbon nanotubes (MWNTs) are similar,but comprise more than one concentric layer of carbon forming the tube.It has been suggested that aligned carbon nanotubes may have good fieldemission properties because they have higher geometric field enhancement(Wang et al., Appl. Phys. Lett, vol. 72, p. 2912, 1998). CNTs can beproduced by chemical vapor deposition (CVD) (Nikolaev et al., Chem.Phys. Lett., vol. 313, p. 91, 1999; Huang et al., Appl. Phys. A, vol.74, p. 387, 2002), arc discharge (Journet et al., Nature, vol. 388, p.756, 1997), laser ablation (Thess et al., Science, vol. 273, p. 483,1997), and other techniques (e.g., Derycke et al., Nano Letters, vol. 2(10), p. 1043, 2002). Additionally, vertically-aligned CNTs can be grownon substrates possessing nanoscale metal catalysts using CVD methods(Huang et al., 2002) at temperatures from about 550° C. to about 1200°C.

All of the abovementioned techniques, however, have poor growthuniformity and none can practically deposit carbon nanotubes over largeareas. Furthermore, the growth conditions require relatively hightemperatures, which impede their utilization with low-temperature andgenerally inexpensive substrate materials.

Another problem with using the abovementioned CNT growth techniques forgenerating the cathode material for field emission displays is that thedensity of the CNTs produced may be too high. Researchers have foundevidence that the field emission properties of high density CNT cathodesis less than expected because the neighboring nanotubes shield theextracted electric fields from each other (Bonard et al., AdvancedMaterials, vol. 13, p. 184, 2001). As a result, high-resolutionlithography has been employed to control CNT density by creatingcatalytic dots capable of growing CNTs (Huang et al., Appl. Phys. A,vol. 74, p. 387, 2002). This method is very expensive, however, andrequires growth on high-temperature substrates.

Thus, there is a demonstrated need to be able to harvest fabricated CNTsand apply or dispense them onto various substrate materials at lowtemperatures. There is also a need to be able to control the density ofthe CNTs in an effort to optimize their field emission properties.

SUMMARY OF THE INVENTION

The present invention is directed toward a new cathode for fieldemission devices, methods for making such a cathode, and methods foroptimizing the electron field emission performance of such a cathode bylowering the threshold field of emission and increasing emissioncurrent. Such a cathode comprises a cathode material, which in turncomprises carbon nanotubes (CNTs) and particles. Optimization of theelectron field emission performance is accomplished by modulating thedensity of the field emitters (CNTs) within a particulate matrixmaterial. It is believed that the optimal concentration of CNT fibers inthe cathode material mixture (CNTs and particles) is that which leavesthe highest number of CNTs available for emission, but not so high thatthey interfere with the performance of each other through electricalshielding of the applied field. Furthermore, such a mixture can beapplied to a very wide range of materials since the processing can bedone at room temperature and since the optimization of CNT concentrationis substrate-independent. This method is also very economical in that nohigh-resolution lithography processing step is required. It is likelythat any application involving the use of CNT materials as fieldemitters could potentially benefit from this invention.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a cathode comprising CNTs and particles;

FIG. 2 illustrates a field emission display device incorporating thepresent invention;

FIG. 3 illustrates a ball milling device which can be used to grindCNTs;

FIG. 4 illustrates how spraying can be used to deposit a CNT andparticles mixture on a substrate;

FIG. 5 illustrates a screen printing device, which can be used in thedepositing of a CNT and particles mixture onto a substrate;

FIG. 6 illustrates how dispensing or ink jet printing can be used todeposit a CNT and particle mixture on a substrate;

FIG. 7 illustrates a process whereby a cathode of the present inventionis “activated” by a taping procedure;

FIGS. 8A and B illustrate scanning electron micrographs which contrastmixtures of A) 90 wt. % SWNTs+10 wt. % nanoparticles with B) 10 wt. %SWNTs+90 wt. % nanoparticles;

FIG. 9 illustrates electron field emission I/V curves of cathodescomprising CNTs and alumina nanoparticles;

FIG. 10 illustrates a plot of electric field as a function of CNTconcentration (balance is alumina nanoparticles) for various cathodes ofthe present invention at 25 mA of emission current;

FIG. 11 illustrates electron field emission I/V curves from CNTs mixedwith alumina nanopowders, then activated using a taping process;

FIG. 12 illustrates a plot of electric field as a function of CNTconcentration (balance is alumina nanoparticles) for various cathodes ofthe present invention at 25 mA of emission current, wherein the cathodeshave been activated by a taping process;

FIG. 13 illustrates a swelling of clay particles as they are rehydrated;

FIG. 14 illustrates how shear forces can be used to align mixtures ofCNTs and lamellar (clay) particles;

FIG. 15 illustrates how CNTs can be trapped between the layers of clayparticles as a result of dehydration;

FIG. 16 illustrates the I-V characteristics of a cathode comprising CNTsand clay particles;

FIG. 17 illustrates an image on a phosphor screen generated by a cathodecomprising CNTs and clay particles; and

FIG. 18 illustrates a data processing system configured in accordancewith the present invention.

DETAILED DESCRIPTION

The present invention is directed toward cathodes and cathode materialscomprising carbon nanotubes (CNTs) and particles. The present inventionis also directed toward field emission devices comprising a cathode ofthe present invention, as well as methods for making these cathodes. Insome embodiments, the cathode of the present invention is used in afield emission display. The invention also comprises a method ofdepositing a layer of CNTs and particles onto a substrate to form acathode of the present invention, as well as a method of controlling thedensity of CNTs used in this mixed layer in an effort to optimize thefield emission properties of the resulting layer for field emissiondisplay applications.

CNTs, according to the present invention, can be single-wall carbonnanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), double-wallcarbon nanotubes, buckytubes, carbon fibrils, and combinations thereof.Such CNTs can be made by any known technique, and they can be optionallypurified. Such CNTs can be metallic, semiconducting, semimetallic, andcombinations thereof. In some embodiments, the CNTs are chemicallymodified and/or derivatized. In some embodiments, the CNTs aremetallized according to the techniques described in commonly-assignedand co-pending U.S. patent application Ser. No. 10/406,928, filed Apr.4, 2003, and incorporated herein by reference.

The particles with which the carbon nanotubes are mixed can be of anymaterial which serves to suitably reduce the density of the CNT materialwithin the cathode so as to effectively enhance the field emissionproperties of the cathode when integrated into a field emission device.Such particles include, but are not limited to, spherical particles,disk-shaped particles, lamellar particles, rod-like particles, andcombinations thereof. The material of such particles can be conductive,semiconducting, insulating, or combinations thereof. Such materials caninclude metals, alloys, polymers, semiconductors, dielectrics, clays,and ceramics. Dielectric materials that can be used include, but are notlimited to, Al₂O₃, CeO₂, La₂O₃, TiO₂, SiO₂, TiC, WC, glass frit,diamond, and combinations thereof. Semiconductor materials that can beused include, but are not limited to, Si, GaAs, GaN, and combinationsthereof. Metals that can be used include, but are not limited to,nickel, iron, chromium, alloys, and combinations thereof. Theseparticles function as a matrix material for the CNTs and effectivelyreduce the interaction between CNTs with a consequence of enhancingfield emission properties. Such particles can vary in size and shape,but generally have diameters which range from about 1 nanometer (nm) tohundreds of micrometers (μm).

In some embodiments of the present invention, the particles may alsofunction to trap or hold the CNTs onto a substrate or in a CNT-particlematrix. As will be described later, some particles can be porous. Otherparticles, such as clays, can be layered—with gaps between the layers.These gaps can be dependent on the state of the clay. For example, ifthe clay is fully hydrated or saturated with molecules between thelayers, the gaps can be several nanometers wide. CNTs or functionalizedCNTs may enter the pores or gaps in the particles. This alone may besufficient to hold or capture the CNTs. Furthermore, the hydrates ormolecules between the layers in the particles may be taken out withcertain processes such as heating and/or drying. This process cancollapse the layers of the particles, further holding or capturing theCNTs.

A cathode comprising CNTs and particles is shown, as a typicalembodiment, in FIG. 1. Referring to FIG. 1, the cathode comprises asubstrate 103 on which a cathode material 106 is in contact. The cathodematerial comprises CNTs 105 and particles 104. In some embodiments, thesubstrate 103 is a glass base 101 supporting a conductive layer 102.

An embodiment wherein the cathode of the present invention isincorporated into a field emission display device is shown in FIG. 2.Referring to FIG. 2, the cathode described above can be incorporatedinto field emission display 200. On base 101 is deposited conductivelayer 102 on which the cathode material 106 is deposited. The anodeincludes substrate 204, which may be a glass substrate 204, conductivelayer 205, which may be ITO, and a phosphor layer 206 for receivingelectrons emitted from the cathode material layer 106. Electrons areemitted from layer 106 in response to an appropriate electric fieldbetween the anode and the cathode.

FIG. 2 shows a very simplified view of a display. Not shown in the FIG.2 are the side walls that complete the enclosure of the gap between theanode and cathode. Also not shown are spacers that hold the gap betweenthe anode and the cathode. In normal operation, the gap between theanode and cathode is evacuated to pressures in the range of about 10⁻⁶Torr or better vacuum. Many displays have many independently addressablelines on both the cathode and the anode in order to create pixels andthus form the image on the anode. FIG. 2 also illustrates a diodedisplay architecture. Other display architectures may have 3 (anode,cathode and grid) or more elements. In such cases, the addressing linesand columns are on the cathode and the grid; the anode is held at onepotential. The invention described here is not dependent on theparticular type of field emission display architecture (single pixel ormulti-pixel, diode or triode, color or monochrome, etc.).

The density of the nanotubes in the cathode material is related to theweight of the CNTs relative to the weight of the particles. The weightpercent of the CNTs can vary generally from about 0.1% to about 99%, andmore specifically from about 40% to about 60%.

In some embodiments, the cathode material (CNTs+particles) of thepresent invention is in the form of a layer. Depending on theapplication, this layer can vary in area and thickness. This layer has athickness which ranges generally from about 10 nm to about 1 millimeter(mm), specifically from about 100 nm to about 100 μm, and morespecifically from about 1 μm to about 20 μm.

In some embodiments of the present invention, the cold cathode comprisesa substrate on which the cathode material resides. Such a substrate canvary widely in size and shape, but typically has a flat surface. Thesubstrate can be of any material or combination of materials whichsuitably provides for a substrate according to the present invention.The substrate material can be selected from conductors, semiconductors,insulators, and combinations thereof. In some embodiments, the substratecomprises one or more stacked layers. In some embodiments, glass is usedas the substrate.

In field emission devices, the cathode material of the present inventioncan enhance the field emission process by lowering the electric fieldneeded to extract a current density of a particular value.

The methods by which the cathodes of the present invention are madegenerally comprise the steps of: 1) selecting an appropriate combinationof carbon nanotubes and particles, 2) mixing the carbon nanotubestogether with the particles, and 3) applying the mixture to anappropriate substrate.

Selection of CNTs and particles can vary depending on the desiredapplication and method of processing used. Cost considerations can alsoplay a role.

In some embodiments, the CNTs and/or particles are ground prior tomixing. In some embodiments, this is an integral part of the mixingprocess. Such grinding can be done using a variety of methods, such aswith a ball-milling device as shown in FIG. 3. Referring to FIG. 3, aball-milling device 300 comprises a motor 301 to which a wheel 302 isattached to a belt 303 which drives a second wheel 304. This secondwheel 304, via a turbine 305, gear 306, and chain 307 assembly, drives ashaft 308 which spins a milling chamber 309. It is in this millingchamber 309 that the CNTs and/or particles are placed.

Mixing the CNTs and particles can be done in a variety of ways. In someembodiments, the CNTs and particles are dry mixed. In some embodiments,the particles and/or the CNTs are separately pre-dispersed.Pre-dispersion, according to the present invention, can involvesuspension and/or dispersal in a solvent. Solvents can be any solvent orsolvents which suitably disperses the CNTs and/or particles, accordingto the present invention. Such solvents include, but are not limited to,water, isopropyl alcohol (IPA), methanol, ethanol, tetrahydrofuran(THF), CH₂Cl₂, cyclohexane, and combinations thereof. In general, it isadvantageous in most embodiments of the present invention for thesolvent to be easily removed (as in evaporation). In some embodiments,ultrasonication is used to facilitate the suspension and/or dispersal ofthe CNTs and/or particles in a solvent.

Mixing, according to the present invention, is generally done in such away as to achieve a desired ratio (e.g., weight percent) of CNTs in aparticulate matrix so as to effect an optimum or desired CNT density forfield emission. Such ratios are generally dependent on the materialsused, the particle size, the homogeneity of the mixing, the thickness ofthe mixture layer, etc.

In some embodiments, particles have a lamellar shape and align with CNTsif a shear force is applied. Nanotubes in a mixture can thus be alignedin the same direction so as to effect the CNTs orientation on thecathode to improve field emission performance. More specifically, CNTscan be mixed with clay particles forming a water-based sol or gel.

In some embodiments, additional materials are mixed together with theCNTs and particles. Such additional materials may include, but are notlimited to, binders, surfactants, dispersal agents, and combinationsthereof.

Application of the mixture to a substrate can be accomplished in avariety of ways. Generally, either a pre-formed composite materialcomprising CNTs and particles is contacted with a substrate using acontacting means, or a mixture of CNTs and particles is applied to asubstrate using a deposition means. In some embodiments, the CNT andparticle mixture is first dispersed in a solvent and then deposited onthe substrate, wherein the solvent is subsequently removed. In someembodiments, the CNT and particle mixture is deposited over an area in aparticular arrangement or pattern. In some embodiments, this is doneusing a shadow mask. Such deposition means include, but are not limitedto, spraying, brushing, electrophoretic deposition, dipping, dispensing,screen printing, ink jet printing, spin-coating, and combinationsthereof. In some embodiments, the substrate is heated before, during,and/or after the deposition. Such heating can serve to aid solventremoval.

FIG. 4 illustrates an embodiment wherein the CNT and particle mixture issprayed onto a substrate. A condensed gas 401 is used to charge anatomizer 402 containing a solvent-suspended mixture of CNTs andparticles 403. Mixture 403 is sprayed onto a substrate 404, optionallyin contact with heater 405 and/or infrared (IR) heat lamp 406, to formcathode material layer 407 comprising CNTs and particles.

FIGS. 5A-C illustrate a screen printing method by which CNT and particlemixtures can be deposited onto a substrate according to some embodimentsof the present invention. Referring to FIG. 5A, a substrate 501 isplaced on a substrate stage/chuck 502 and brought in contact with animage screen stencil 503. A paste 504 comprising CNTs and particles isthen “wiped” across the image screen stencil 503 with a squeegee 505, asshown in FIG. 5B. The paste 504 then contacts the substrate 501 only inthe regions directly beneath the openings in the image screen stencil503. The substrate stage/chuck 502 is then lowered to reveal thepatterned cathode material 506 on substrate 501, as shown in FIG. 5C.The patterned substrate is then removed from the substrate stage/chuck.

FIG. 6 illustrates an embodiment wherein a dispensor or an ink jetprinter is used to deposit a CNT and particle mixture onto a substrate.Referring to FIG. 6, printing head 601 is translated over a substrate604 in a desired manner. As it is translated over the substrate 604, theprinting head 601 sprays droplets 602 comprising CNTs and particlesdispersed in a solvent. As these droplets 602 contact substrate 604,they form printed cathode material 603 comprising CNTs and particles. Insome embodiments, the substrate 604 is heated so as to effect rapidevaporation of solvent within said droplets. Heat and/or ultrasonicenergy may be applied to the printing head 601 during dispensing.

In some embodiments, the cathode of the present invention, oncefabricated, is heated in a vacuum environment prior to incorporating itinto a field emission device.

In some embodiments, an activation process is used to activate the layercomprising the CNTs, as described in commonly assigned, co-pending U.S.patent application Ser. No. 10/269,577, filed Oct. 11, 2002,incorporated herein by reference. In some embodiments, this activationprocess comprises a taping process. This taping process is believed toeffect alignment of CNTs at the surface, which is thought to enhancefield emission properties of the cathode. Referring to FIG. 7, acathode, comprising a substrate 702 and a cathode material 701 isprovided in step 7001. The cathode material comprises CNTs andparticles. In step 7002, a tape 703 is placed on top of the cathodematerial 701 with adhesive side toward the cathode material 701. In step7003, tape 703 is removed to yield an activated layer comprisingoriented CNTs 704.

In embodiments utilizing metallized CNTs, such metallized CNTs can bedeposited in an aligning field or subjected to an aligning field afterdeposition, such as according to procedures put forth in commonlyassigned, co-pending U.S. patent application Ser. No. 10/406,928,incorporated herein by reference.

Control over the density of CNTs in a deposited film (e.g.,CNTs+particle layer) is achieved by varying the ratio of CNT materialsto the particle powders. Optimization of this results in an improvementof the field emission properties of the deposited film by lowering theelectric field required to extract electron emission from the film. Nohigh temperature processing steps are needed in this invention, allprocesses can be carried out at, or near, room temperature.

The following examples are provided to more fully illustrate some of theembodiments of the present invention. The examples illustrate methods bywhich CNTs are mixed with particles and the resulting compositionincorporated into a field emission devices as a cathode material. Itshould be appreciated by those of skill in the art that the techniquesdisclosed in the examples which follow represent techniques discoveredby the inventors to function well in the practice of the invention, andcan thus be considered to constitute exemplary modes for its practice.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments which are disclosed and still obtain a like or similarresult without departing from the spirit and scope of the invention.

EXAMPLES Example 1

This example illustrates an embodiment of the present invention, whereinSWNTs are mixed with alumina (Al₂O₃) nanoparticles to achieve anappropriate density, and then applied to a substrate and used as thecathode in a field emission display (see FIGS. 2 and 13). A detailedexample of this embodiment is given in an effort to better illustratethe invention.

SWNTs were obtained from CarboLex, Inc., Lexington, Ky., USA. TheseSWNTs were about 1-2 nm in diameter and about 5-20 μm in length. For thepurposes of this example, it will be understood that SWNTs are simply asubset of CNTs and that in many cases the terms can be usedinterchangeably. Al₂O₃ nanoparticles were obtained from Alfa Aesar, WardHill, Mass., USA. The nanoparticles are as small as 10-20 nm with apurity of 99.98%.

A simple ball mill was used to grind SWNT bundles. FIG. 3 is anillustrative diagram of this ball mill. The milling rate of this machineis on the order of about 50-60 revolutions per minute. In this method, 1g of SWNTs, along with tens of Al₂O₃ balls used for grinding (5-10 mm indiameter) were mixed with 200-300 ml isopropyl alcohol (IPA). Thematerial was ground for 1-14 days in order to disperse the carbonnanotubes. Note that in some embodiments, a surfactant (about 1 drop per100 ml IPA) or similar material was added to the mixture in order toachieve better dispersion of the carbon nanotubes.

As Al₂O₃ nanoparticles easily cluster together, it is helpful todisperse them before they are mixed with the SWNTs. Accordingly, 1 g ofAl₂O₃ nanoparticles were mixed in 200-300 ml of IPA in a beaker withstirring provided by a magnetic stir bar actuated by a hotplate/magnetic stirrer. The material was stirred 1-24 hours so that thenanoparticles could be separated from each other.

The ground SWNTs and dispersed Al₂O₃ nanoparticles were mixed accordingto different weight ratios: 10 wt. % SWNTs+90 wt. % nanoparticles, 25wt. % SWNTs+75 wt. % nanoparticles, 50 wt. % SWNTs+50 wt. %nanoparticles, 75 wt. % SWNTs+25 wt. % nanoparticles, and 90 wt. %SWNTs+10 wt. % nanoparticles. Because SWNTs and nanoparticles easilyclump together if they are not continuously agitated, an ultrasonic hornor bath was used to redisperse them in IPA solution immediately prior tospraying them onto the substrates. In this experiment, the mixture wassprayed onto conductive ITO (indium tin oxide)-coated glass. The mixturewas sprayed onto the substrate with an area of 2×2 cm². In order toachieve better coating uniformity and dispersion on the substrate,additional IPA can be added to the above solution before spraying. Inthis experiment, the solution used for spraying was comprised of about0.1 g of the nanotube/nanoparticle mixture in 100 ml IPA. In order toprevent the IPA from flowing to undesired regions of the substrates, thesubstrates were heated to approximately 70° C., both on the front andback side during the spraying process in an effort to evaporate the IPAquickly. The substrates were sprayed back and forth or up and downseveral to tens of times until the entire surface was coated with themixture. The resulting thickness of the mixture was about 1-20 μm. Thesubstrates were then allowed to dry in air. Shown in FIG. 8 are scanningelectron micrographs which contrast a) 90 wt. % SWNTs+10 wt. %nanoparticles with b) 10 wt. % SWNTs+90 wt. % nanoparticles. FIG. 4illustrates the spraying process.

To compare field emission properties, 100 wt. % SWNTs, without any Al₂O₃nanoparticles, were also sprayed onto the ITO glass substrate. Allcathodes were then tested by mounting them, together with a phosphorscreen, in a diode configuration, like that shown in FIG. 2, with a gapof about 0.5 mm between the anode and cathode. The test assembly wasplaced in a vacuum chamber and pumped to 10⁻⁷ Torr. The electricalproperties of the cathode were then measured by applying a negative,pulsed voltage (AC) to the cathode and holding the anode at groundpotential and measuring the current at the anode (a DC potential couldalso be used for the testing). A graph of the emission current versuselectric field for some samples is shown in FIG. 9.

It can be seen from FIG. 10 that as the concentration of the CNTmaterial decreases, the electric field (as measured by the voltagebetween anode and cathode divided by the gap between anode and cathode)needed to extract a current density of 6.25 mA/cm² also decreases. Thebest concentration of SWNTs in the nanotube/nanoparticle mixture isbetween 20% and 60%. Thus, by lowering the concentration, the density ofCNT material decreases to the point at which the CNTs no longer shieldeach other. One can also see that when the CNT concentration decreasesbelow 20%, the required field increases. This is probably a result ofthe CNT concentration becoming too small and there being too few CNTfibers available to emit electrons.

While not intending to be bound by theory, it is believed that if theCNT concentration is too high, the carbon nanotube fibers electricallyshield the applied field from one another. Ideally, the distance betweenthe tubes, if they were all perfectly aligned, would be about the samedistance as the length of the tube. As the density continues todecrease, the electrical shielding issue does not improve significantly,but the number of fibers available for emitting electrons continues todecrease with the density. Thus, there should be an optimal density oftubes that gives the best emission parameters. The data shown hereindicates that this optimum density is realized with a mixture of CNTmaterial and alumina nanoparticles that is in the range of about 20%-60%weight concentration of CNT material.

Example 2

This example illustrates how activation processes are used to furtherenhance the field emission properties of the cathode of the presentinvention.

In an earlier disclosure (commonly assigned, co-pending U.S. patentapplication Ser. No. 10/269,577, filed Oct. 11, 2002, incorporatedherein by reference), a process of “activating” the CNT film by applyingan adhesive tape material to the film and then pealing the adhesive tapeaway was described. This also has the effect of lowering the density ofthe CNT fibers. This activation process was tried in combination withthe CNT/alumina powder mixture described above. The results wereconsistent.

After a mixture of SWNTs and Al₂O₃ is sprayed onto a substrate, anadhesive tape process was used to remove the top layer of the materialson the surface. In this method, clear tape was used to remove themixture, although it is likely that there are many brands and varietiesof adhesive tape that can be used with similar results. The tape wascontacted to the coating using a laminating process in which theadhesive side of the tape touches both the carbon nanotubes and thealumina particles. Care is taken to ensure that there is no air betweenthe tape and the SWNTs and Al₂O₃ particle coating (if a bubble ispresent, the mixture at that area will not be uniformly removed). Arubber roller is used to further press the tape in order to eliminateair at the intersection between the tape and the mixture coating.Finally, the tape is removed with the result that less than 50% of themixture is left on the substrate. This taping process is illustrated inFIG. 7.

Activated samples were tested for their emission current versus electricfield (I/V) properties in the same manner that the unactivated sampleswere. FIG. 11 shows some of the I/V results of this experiment. FIG. 12plots the electric field when each of the samples reached 25 mA ofemission current (see FIG. 11) versus the CNT concentration as apercentage of the weight of CNT material. It is interesting to note thatfor the samples treated with adhesive tape, the concentration yieldingoptimal performance has shifted to a range of 40-80 wt. % of CNT in theCNT+Al₂O₃ mixture. Since the tape activation process lowers theconcentration of the CNT material as described earlier, this results inthe shifting of the optimal concentration range to higher initialconcentrations of CNT fibers in the Al₂O₃ nanoparticle powder, as seenin FIG. 12.

Example 3

This example illustrates some embodiments of the present invention whichutilize clay particles.

As stated earlier, particles may be nanoparticles and they may alsoinclude porous materials such as porous silicon or one of severalvarieties of zeolite minerals. These particles may also include layeredmaterials, such as clay particles. Examples of clays include, but arenot limited to laponite, bentonite or hectorite. Clays are layeredmaterials with spaces between the layers that can absorb water moleculesor positive and negative ions (cations or anions) and undergo exchangeinteraction of these ions with solvents. Clays have very uniqueproperties. When they are dried, the molecules or ions between thelayers can come out, the gaps between the layers can close and the layerstack can shrink significantly. Correspondingly, when the clay particlesare rehydrated, as shown in FIG. 13, the space between the clay particlelayers 1301 expands.

Clay particles dispersed in solvents such as water can significantlychange the viscosity of the solution. While there are other materialsthat will thicken waterborne solutions, clay particles are unique inthat the viscosity is shear sensitive (shear forces will lower theviscosity by orders of magnitude).

Clay particles may have several advantages over other particles in aCNT+nanoparticle composite. When extruded through tubes or pipes, theclay molecules may also align with each other by shear interaction withthe wall of the extruder. If CNTs are included in a solution of clayparticles, it is possible that the alignment of the clay particles willalign the carbon tubes in a preferred direction or within a preferredplane or layer as illustrated in FIG. 14, wherein layered clay particles1401 and CNTs 1402 flow from a region 1403 where they are randomlyoriented, through a region 1404, and into a region 1405 where they areoriented.

Clay particles may also help trap or capture CNTs between their layersduring the process of dehydration or shrinking of the layers. The gapsbetween the layers can range from about a few nanometers to tens ofnanometers, sufficiently large so as to allow CNTs to penetrate intothem. As the gap shrinks, the CNT may be captured between the layers ofthe clay particles and thus anchored to the particle, as shown in FIG.15, wherein a CNT 1501 is trapped between clay particle layers 1301.This may add greatly to the stability of the CNT+nanoparticle composite.In some embodiments, the CNT fiber may require functionalization to aidthe penetration of the CNT fiber between the layers. Since the clayparticles are flat, pancake-like particles, it is easy for CNTs to betrapped or captured between particles when the solution is dried.

An additional advantage that clay particles have over other particles ornanoparticles is that they can thicken the solution that will be used asa paste for screen printing or as an ink for dispensing or ink jetprinting without adding organics or materials that can outgas in avacuum environment. As a water-based material, it is moreenvironmentally-friendly than other screen printing pastes or inks. Asnoted earlier, clay solutions will flow well under sheer force, but canset to a gel consistency when stationary. This makes for an ideal screenprinting paste since it can easily be spread or “wiped” across thescreen using a squeegee (see FIG. 5), but sets to a gel and does notflow after printing. This can greatly improve the resolution of theprinted pattern. As a dispensing ink (see FIG. 6), it has similaradvantages. It can flow in the dispensing tube with low viscosity,aligning the particles in the process, but once dispensed on thesubstrate, it sets to a gel consistency that does not flow. Since thesolution is water based, the printed gel can easily be dried with heat(on the order of about 100° C.) to reduce it to clay particles and CNTfibers. Unlike most screen printing pastes, this material does notrequire firing at higher temperatures.

A simple experiment was performed to demonstrate that clay particlesmixed with CNTs can be screen printed and that high quality fieldemission results from the process. A quantity of 5.6 grams of clay gel(0.05% to 3% laponite from Southern Clay Products, in water) and 0.1gram of CNTs were ground in a mortar to make CNT ink. A screen printerwas used to print this ink onto an ITO glass substrate as describedearlier (FIG. 5). The sample was dried in an oven at 100° C. for half anhour. Activation with a tape layer may be used to improve the fieldemission properties (described earlier). The I-V characteristics areshown in FIG. 16. The size of the substrate was 3 cm×3 cm.

An image on a phosphor screen as a result of the electron currentemitted from the clay particle+CNT composite cathode material is shownin FIG. 17. Referring to FIG. 17, the individual sub-pixels in thisfigure are 6 mm long by about 1 mm wide. This demonstrates that pixelfeatures can be printed with screen printing techniques, yet requireonly low temperature processing steps to cure and prepare for operationin a field emission device or display.

Example 4

While numerous other applications exist, this example serves toillustrate how a field emission display device comprising a cathode ofthe present invention can be integrated into a data processing system.

A representative hardware environment for practicing the presentinvention is depicted in FIG. 18, which illustrates an exemplaryhardware configuration of data processing system 1813 in accordance withthe subject invention having central processing unit (CPU) 1810, such asa conventional microprocessor, and a number of other unitsinterconnected via system bus 1812. Data processing system 1813 includesrandom access memory (RAM) 1814, read only memory (ROM) 1816, andinput/output (I/O) adapter 1818 for connecting peripheral devices suchas disk units 1820 and tape drives 1840 to bus 1812, user interfaceadapter 1822 for connecting keyboard 1824, mouse 1826, and/or other userinterface devices such as a touch screen device (not shown) to bus 1812,communication adapter 1834 for connecting data processing system 1813 toa data processing network, and display adapter 1836 for connecting bus1812 to display device 200. CPU 1810 may include other circuitry notshown herein, which will include circuitry commonly found within amicroprocessor, e.g., execution unit, bus interface unit, arithmeticlogic unit, etc. CPU 1810 may also reside on a single integratedcircuit.

It should be noted that all of the embodiments described herein can beused to create the display in system 1813.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A method comprising the steps of: a) forming a mixture of carbonnanotubes and particles; and b) depositing a layer of the mixture ofcarbon nanotubes and particles onto a substrate to form a cathode. 2.The method of claim 1, wherein the nanotubes are selected from the groupconsisting of single-wall carbon nanotubes, double-wall carbonnanotubes, multi-wall carbon nanotubes, buckytubes, carbon fibrils,chemically-modified carbon nanotubes, derivatized carbon nanotubes,metallic carbon nanotubes, semiconducting carbon nanotubes, metallizedcarbon nanotubes, and combinations thereof.
 3. The method of claim 1,wherein the particles are selected from the group consisting ofspherical particles, disk-shaped particles, lamellar particles, rod-likeparticles, metal particles, semiconductor particles, polymericparticles, ceramic particles, dielectric particles, clay particles,fibers, nanoparticles, and combinations thereof.
 4. The method of claim1, wherein the step of forming a mixture of carbon nanotubes andparticles comprises a milling operation.
 5. The method of claim 1,wherein the step of forming a mixture of carbon nanotubes and particlescomprises solvent dispersal.
 6. The method of claim 1, wherein themixture of carbon nanotubes and particles is deposited using a methodselected from the group consisting of spraying, brushing,electrophoretic deposition, dipping, dispensing, screen printing, inkjet printing, and combinations thereof.
 7. The method of claim 6,further comprising a step to remove the solvent from the mixture afterdepositing the mixture on the substrate.
 8. The method of claim 1,further comprising a taping process to activate the cathode.
 9. Themethod of claim 1, further comprising a method of aligning the carbonnanotubes within the layer of carbon nanotubes and particles.
 10. Themethod of claim 1, wherein the particles are lamellar.
 11. The method ofclaim 10, further comprising a method of aligning the carbon nanotubesusing a shear force applied to the mixture of the carbon nanotubes andlamellar particles.
 12. The method of claim 10, wherein the lamellarparticles comprise clay.